Electroretinographic (ERG) recordings are physiologic measurements of the eye (human or animal) in response to optical stimulation. Electrodes placed on or near the eye can sense the neurological activity of the retina. The potentials so recovered are very small, ranging from a few micro-volts to a few hundred micro-volts. By analysis of the signals, disease or injury of the eye can be diagnosed.
The International Society for Clinical Electrophysiology of Vision (ISCEV), in its “Standard for full-field clinical electroretinography” provides recommendations for the use and types of electrodes. Conductive fibers, corneal wicks, foils, conjunctival loops, and contact lens types are referenced.
Contact lens electrodes are known to provide the highest amplitude and provide the most stable or repeatable results. The greatest signal is found at the center, and gradually lessens toward the periphery of the eye. But corneal electrodes can be painful, and too difficult for some subjects.
The means for making electrodes had proceeded on an ad hoc basis since its earliest beginnings. Most of the pioneering development was conducted in research centers, and as the value of the methods to diagnose retinal disease was recognized, it has become an important clinical tool.
Examples of ERG electrodes that relate to this design are:                Described as a transparent corneal cup, it is used in conjunction with a conductive fluid (U.S. Pat. No. 4,131,113).        Owing to its simplicity and economy, a corneal wick described by W W Dawson, G L Trick and C A Litzkow, is popularly referred to as the “DTL electrode” (U.S. Pat. No. 4,417,581).        Not quite a contact lens, because it can have a central aperture hole, a conductive gel corneal electrode is cast to shape with an attached flexible wire lead (U.S. Pat. No. 4,735,207).        At the far end of this category, scleral electrodes record biopotentials as far from the cornea to avoid issues of damage or irritation (U.S. Pat. No. 4,874,237).        A second type of scleral electrode was described that is formed of a stiff coaxial cable loop that is inserted under the lower eyelid (U.S. Pat. No. 5,154,174).        Combining both corneal and scleral electrode support in a large contact lens, a device that also provides a fixture for the illumination optics is useful for animal studies because it prevents blinking (U.S. Pat. No. 5,297,554).        An array of electrodes in a corneal lens combined with multifocal ERG is applied in a retinal mapping application (U.S. Pat. No. 7,384,145).        Combining a flexible coaxially shielded interconnecting cable and a spherical button electrode, the assembly can be packaged individually, or in an array. The purpose of the spherical form of the electrode is to help anchor it in one spot (U.S. Pat. No. 7,596,400).        
When corneal electrodes are used, topical anesthesia may be required as the cornea is very sensitive. It may also be necessary to apply a lubricating solution between the cornea and the electrode.
Coaxial cables are useful to shield the voltage measurement from outside interference. However, coaxial cables are prone to a source of noise that is piezo and triboelectrically generated. As the cable is flexed, the dielectric between the center conductor and the shield is alternately squeezed or stretched, electrical charges are generated. Special low noise cables may have a graphite inner lining to reduce triboelectric noise, but the piezo effects can remain.
To limit the flexing of the cables, attachment of the lead wires near the eye is necessary. This has to be done with great care to avoid pulling the electrode(s) out of alignment. This is in spite of movement of the eye or blinking. This can be very uncomfortable to the subject and painful to remove.